Condition sensing apparatus



TO A CIRCUIT Dec. 5, 1961 w. B. HAMELlNK 3,012,146

CONDITION SENSING APPARATUS Filed June 30, 1958 INVENTOR.

WILLIAM B. HAMELINK United States Patent" C) F 3,012,146 CONDITION SENSING APPARATUS William B. Hamelink, Minneapolis, Minn., assignor to lVIinneapolis-Honeywell Regulator Company, Minneapolis, Minn, a corporation of Delaware Filed June 30, 1958, Ser. No. 745,532 Claims. (Cl. 25083.6)

This invention is concerned with an improved condition sensing apparatus and more particularly with a condition sensin apparatus having an improved safe start arrangement.

Specifically, the preferred modification of the present invention discloses a condition sensing apparatus utilizing a Geiger tube as the condition sensing element and having associated means, including an integrating network, to provide means to distinguish an actual condition to be detected and the inherent background count of the Geiger tube, and likewise having a main output means and a safe start means. Such a condition detecting apparatus may be used, for example, to detect the presence of flame at a fuel burner unit' which is being controlled.

The teachings of the prior art disclose the advantages of providing a safe start arrangement which function such that upon initial energization of the flame or condition sensing apparatus, the ability of the condition sensor to detect the presence of flame is checked by simulating the presence of flame and causing the main output relay, also referred to as the flame relay, to be energized in response to this simulated presence of flame. This initial energization of the flame relay causes a safe start relay to be energized. The safe start relay is provided with means to maintain itself energized once it is placed in the energized condition. Furthermore, the safe start relay controls the means which initially simulated the presence of flame and therefore the presence of flame is no longer simulated and the flame relay becomes de-energized.

in this manner, upon initial energization of the condition sensing apparatus, the ability of the apparatus to respond first to the simulating presence of flame and second to the actual absence of flame is checked. The switch means of the flame relay and the safe start relay are so interlocked with further control circuits that components of the fuel burner unit itself, such as a fuel valve, may not be energized until the above described safe start sequence has been completed.

The present invention is directed to this general type of safe start arrangement and particularly to one in which the condition sensing means comprises a pair of electrodes disposed in an ionizable gaseous medium, of which a Geiger tube is an example. As is well known, a Geiger tube has an inherent background count or counting rate which occurs at random and rather infrequent intervals. This background count is caused by external mechanisms, such as stray radiation which may enter the area being supervised by the Geiger tube. In order to distinguish between this random count and a sustained counting rate which is indicative of the presence of the condition to be detected, the prior art teaches the use of signal storage or integrating means which require that a sustained counting rate be experienced before an output signal is derived to control further means.

The present invention is concerned with such a system wherein the present invention provides means to initially render the signals storage or integrating means inoperative such that the inherent background count of the Geiger tube produces the output signal normally present only with a sustained counting rate, thereby simulating the condition to be detected.

t is therefore an object of the present invention to provide an improved condition sensing apparatus having a condition sensing device of the type including a pair of 3,012,146 Patented Dec. 5, 1961 r'* ce electrodes disposed in an ionizable gaseous medium and having a signal transmission network including integrating means such that an output voltage appears at the output of the network only upon the sensing means experi' encing a condition to be detected. Furthermore, means are provided to render the integrating means inoperative such that the output means is initially energized in response to the background countof the sensing means but is subsequentlyenergized only upon the sensing means sensing the condition to be detected;

It is a further object of the present invention to provide an improved condition sensing apparatus having a Geiger tube which is connected to a source of voltage such that a signal is produced at a random frequency due to the background count of the Geiger tube and of a sustained frequency when the Geiger tube is subjected to a condition to be detected. Signal network means are provided having electrical storage means such that an output normally appears therefrom only upon said Geiger tube being subjected to the condition to be detected. Furthermore, the output of the network energizes a safe start means which then maintains itself energized and which safe start means controls means to render the electrical storage means operative to thereby cause an output means to be initially energized due to the background count of the Geiger tube and to be subsequently energized only due to a sustained counting rate indicative of the presence of the condition to be detected.

These and other objects of the present invention will be apparent to those skilled in the art upon reference to the following specification, claims, and drawing of which the single figure is a schematic representation of the preferred modification of the present invention.

Referring to the single figure, the reference numeral 10 designates a condition sensor in the form of a Geiger tube having a pair of electrodes 11 and 12 disposed in an ionizable gaseous medium electrode 11 being the anode and electrode 12 being'the cathode. The electrodes 11 and 12 are connected to conductors 13 and 14 respectively and these conductors are connected to a source of voltage in the form of a multi-vibrator quenching circuit designated generally by means of the reference numeral 15. The multivibrator quenching circuit 15 and the manner in which it functions and cooperates with Geiger tube 10 is described and claimed in the co-pending application of William B. Hamelink, Serial No. 729,710, filed April 17, 1958.

The output of the Geiger tube 18 which, is a random signal pulse in the case of the background count or a sustained counting signal in the case of detection of a condition, is applied to a voltage doubler network including a capacitor 16 and a diode 17, the voltage doubler network being identified generally by means of the reference numeral 18.

The output of the voltage doubler network 18 is applied to a network 19 which distinguishes between a random signal or a sustained counting signal and accomplishes this in part by means of an integrating network. The structure of the network 19 is disclosed and claimed in the co-pending Fred T. Deziel application, Serial No. 707,791, now Patent No. 2,919,438, filed January 8, 1958.

The output of the network 19 is applied to an output electronic stage 20 which is connected to control the energization of a main output relay 21 having a winding 22 and switches 23 and 24. Relay 21 is shown in a deenergized condition wherein switch 23 is open and switch 24 is closed.

The switch '23 of relay 21 is connected in an energizing circuit for a safe start relay 25 having a winding 26 and switchesj27, 28 and 29. The relay 25 is shown in the de-energized condition wherein the switches 27 and 29 are open and the switch 28 is closed. As will be de- 3 scribed in greater detail, with the apparatus as shown in the single figure, the switch 28 of relay 25 is connected to render the integrating means of network 19 inoperative such that upon closing of a master switch 88, relay 21 is initially energized in response to the inherent background count of the Geiger tube 10. Energization of the relay 21 closes switch 23 which thereby energizes the winding 26 of relay 25. Energization of relay 25 closes switch 27 to maintain its winding 26 energized independent of the state of energization of relay 21. Energization of relay 25 also opens the switch 28 to thereby render the integrating means of network 19 operative such that relay 21 no longer is energized in response to the inherent background of Geiger tube and therefore 21 becomes de-' trol electrode 34 and 35, cathode 36 and 37, cathode heater 38 and 39. Operating voltage for the discharge devices and.3-1 is derived from a transformer 40 having a primary winding 41 connected to a source of alternating voltage by means of conductors 42 and 43 and master switch 88. Transformer 46 is provided with secondary windings 44, and 46. The secondary winding 45 is connected to a rectifier 47 and a capacitor 48 to form a DC. voltage supply means which is bridged by a 'blecder network including resistors 49, 50 and 51. The lower or negative plate of capacitor 48 is connected by means of conductor 52 directly to the cathode 39 of discharge device 31 and through a resistor 53 to the cathode 36 of discharge device 30. The upper or positive plate of capacitor 48 is connected by means of conductor 54 and resistor 55 to the anode of discharge device 51 and by means of resistor 56 to the anode of discharge device 30.

The cathode 36 of discharge device 30 is also connected to the positive conductor 54 by means of a resistor 56 and its control electrode 34 is connected through a resistor 58 to the negative conductor 52. Such a construction determines the stable condition of operation of discharge device 30 wherein this discharge device is normally non-conducting, thereby placing the potential level of the lower terminal of its anode load resistor 56, that is terminal 58, at a high positive potential level, approximately equal to that of conductor 54.

The, control electrode 35 of discharge device 31 is connected to the positive conductor 54 through a resistor 59. This connection determines the stable operating condition of discharge device 31 wherein this discharge device is conductive. Therefore, the lower terminal of its anode load resistor 55, that is terminal 60, is at a substantially lower potential level than the potential of conductor 54. In this manner, the stable operating condition of the multi-vibrator quenching circuit 15, that is discharge device 30 is non-conductive and discharge device 31 is conductive, applies an operating voltage to the electrodes 11 and 12 of Geiger tube 10. As is explained more fully in the above mentioned William B. Hamelink co-pending application directed to this .multi-vibrator quenching circuit, the ionization of Geiger tube 10 by means of an ionizing event passing through the tube, this being either a random ionizing event or an ionizing event due to the presence of a condition, causes the multi-vibrator circuit to be pulsed from the stable state to an unstable state wherein discharge device 30 is conductive and discharge 31 is non-conductive. This then appreciably changes the c 4 I voltage applied to the Geiger tube 10 and therefore quenches the tube, allowing the multi-vibrator quench circuit 15 to once again return to its stable condition wherein Geiger tube 10 is again sensitive to the presence of an ionizing condition.

In this manner, the potential level of terminal 60 varies from a low to a relatively high level and for each cycle of operation a pulse of voltage is supplied through a resistor 61 to capacitor 16 and diode 17. Capacitor 16 effectively changes this pulse of voltage to an AC. voltage, onehalf cycle of which is rectified by means of diode 17 to charge capacitor 16. This charge then adds to the opposite one-half cycle of the AC. voltage to provide a voltage doubler action, applying a voltage between conductors 5'2 and conductor 62.

The voltage between conductors 52 and 62 is applied to a pair of voltage dividing networks, one network including resistors 63 and 64 and the other network including resistors 65 and 66. The junction of resistors 63 and 64 is connected to a control electrode 67 of a discharge device 68. The junction of resistors 65 and 66 is connected to the control electrode 69 of discharge device 68. Operating voltage for discharge device 68 is derived by means of a resistor 70 which connects anode 71 to the positive conductor 54 and by means of conductors 72and 73 which connect cathode 74 to the terminal joining resistors 50 and 51.

As explained more completely in the above mentioned Fred T. Deziel co-pending application, the signal voltage applied to the control grid 67 is applied directly thereto while the signal applied to the control grid 69 is applied through means including an integrating network; The integrating network includes a capacitor 75 which is connected to control electrode 69 and to cathode 74. Within the teachings of the above mentioned Fred T. Deziel co-pending application, the electronic stage identified by the reference numeral 19 is effective to discriminate between the background count of the Geiger tube 10 and a sustained counting rate indicative of the presence of the condition to be detected. It suffices to say for the purposes of the present explanation that a random background count causes a voltage to appear immediately at control electrode 67. However, the voltage applied to control electrode 69 is integrated or delayed by means including capacitor 75 and it is only with a sustained counting rate that a coincidence of signal voltage appears on the control electrodes 67 and 69. In order for discharge device 68 to be rendered conductive, it is necessary that a signal voltage appear on these two electrodes simultaneously, and this occurs only as a result of a sustained counting rate of Geiger tube 10.

Within the teachings of the present invention, the integrating capacitor 75 is shorted by means including condoctors 76 and 77 which connected to switch 28 of the safe start relay 25. With switch 28 closed, the integrating means is rendered inoperative.

The control electrodes 67 and 69 of discharge device 68 are provided with a bias voltage which is derived across resistor 51. The voltage across this resistor is of such a polarity as to bias the electrodes 67 and 69 substantially to cutoff. However, when switch 28 is closed, the control electrode 69 is connected directly to the cathode 74 and therefore the bias is removed from this electrode. As above mentioned, the integrating means including capacitor 75 is also rendered inoperative. Therefore, the background count of Geiger tube 10 will cause discharge device 68 to become conductive.

The output of discharge device 68, that is the voltage developed across its plate load resistor 70, controls the input of a further stage 20 having a discharge device 78 including an anode 79, a control electrode 80, and a cathode 81. The input circuit of discharge device 78 includes capacitor 82 which has its right-hand plate connected by means of a conductor 83 to the junction of resistors 49 and 50. The left-hand plate of capacitor 82 is connected by means of conductor 84 to'the lower terminal of resistor 7 8. Considering the condition wherein discharge device 68 is non-conductive, the lower terminal of resistor 76 is at approximately the potential level of conductor 54 and capacitor 82 is thereby charged such that its right-hand plate is negative with respect to the left-hand plate. This then places a negative bias on the control electrode 89 of discharge device 78 and therefore biases this discharge device to be nouconductive. However, upon discharge device 68 being rendered conductive, either in response to the background count of discharge device 18 when capacitor 75 is operative, or in response to a sustained counting rate of Geiger tube 10 when capacitor 75 is operative, a voltage appears across resistor 70 and therefore capacitor 82 discharges to render discharge device 78 conductive.

Operating voltage for discharge device 78 is derived from the secondary winding 44 which is connected to a rectifier 85 and a capacitor 86. The anode and the cathode circuit of discharge device 78 includes winding 22 of output relay 21 and the conduction of this discharge device operatively energizes the relay winding 22.

Turning now to the operation of the preferred modification of the present invention upon initial starting, the single figure shows the apparatus with the master switch 88 in an open condition and therefore voltage is not applied to the transformer 41 and its associated component. When it is desired to place the apparatus in an operating condition, the switch 88 is first closed. This energizes the primary winding 41 of transformer 40 and by means of a secondary winding 46, the various heaters of the discharge devices 17, 68 and 78 are energized, thereby heating these discharge devices to an operating temperature. Furthermore, the heaters 38 and 39 of the discharge devices 30 and 31 are heated by means of a portion of the secondary winding 45.

The multi-vibrator quench circuit is therefore placed in its stable condition and an operating voltage is applied to the Geiger tube It The relays 21 and are both de-energized and therefore the switch 28 of relay 25 shorts capacitor 75 and renders the integrating means of network 19 inoperative. Within a relatively short period of time Geiger tube 10 experiences an inherent background count and the tube becomes ionized to thereby cause the multi-vibrator quench circuit to move from its stable to its unstable condition and back again to its stable condition.

This applies a pulse of voltage to the voltage doubler network 18 and this voltage is in turn applied simultaneously to control electrodes 67 and 69 of discharge device 68. It will be remembered that at this time the capacitor 75 is shunted by switch 28 of relay 25, thereby rendering the integrating means of network 19 inoperative. The discharge device 68 is thereby rendered conductive and in the manner above explained, the discharge device 78 is rendered conductive to energize relay 21. The energization of relay 21 completes a circuit from power line conductor 43 through the winding 26 of relay 25, conductor 88, switch 23, and conductor 89 to power line conductor 42. Thus, winding 26 is energized. Energization of winding 26 completes a holding circuit for this winding which can be traced from power line conductor 43 through winding 26, switch 27 and conductor 99 to power line conductor 42. In this manner, once relay winding 26 is energized, it maintains itself energized independent of the state of energization of the winding 22 of relay 21. The energization of relay winding 26 causes switch 28 to move to an open condition. This removes the sort circuit from around capacitor 75 and the integrating means of network 19 is rendered operative.

Since the integrating means of network 19 is now operative, the discharge device 68 now responds only to a sustained counting rate of Geiger tube 10 and no longer responds to the inherent background count of the Geiger tube. Therefore, the discharge devices 68 and 78 are rendered nonconductive and relay winding 22 is de-energized.

A specific function of the apparatus shown in the single figure would be to monitor a particular area for a particular condition to be detected, for example to monitor a fire box of a fuel burner unit for the presence or absence of a flame. The above explained cycle of operation is necessary to insure that the apparatus can first sense the simulated presence of flame, this being simulated by means of switch 28 which renders the integrating means inoperative, and to second sense the actual absence of flame which is accomplished by rendering the integrating means operative so the apparatus responds only to a sustained counting rate of Geiger tube 10.

The circuit Which indicates that the apparatus has satisfactorily completed this above described safe start cycle includes the switch 29 of relay 25 and the switch 24 of relay 21. If the apparatus has in fact satisfactorily completed the safe start cycle, a circuit is completed from conductor 91 through switch 29, conductor 92, and switch 24 to conductor 93. This circuit which is completed between conductors 91 and 93 can be used in a number of manners, for example this may be a portion of the starting circuit of a main burner control relay such that the main burner control relay may not be energized upon a call for operation of the fuel burner unit until this portion of the circuit is complete to indicate that the apparatus disclosed has satisfactorily completed the safe start cycle. Furthermore, the output relay 21 as well as the safe start relay 25 may be provided with further switches which control various functions associated with an over all system, for example an over all burner control system.

Once the apparatus has successfully completed the safe start cycle, the apparatus is in a condition to energize relay 21 in response to a sustained counting rate of Geiger tube 10. Therefore, should the apparatus be used to monitor a fuel burner flame, the main burner control relay may be energized by means or" a starting circuit including switches 24 and 29 to establish flame at the fuel burner unit. Geiger tube 10 is then subjected to the radiation from this flame and a sustained counting rate is experienced. Relay 21 is then energized and a further control function may be accomplished by means of other switches of relay 21, not shown.

From the above description it can be seen that I have provided an improved condition sensing apparatus of the type providing a saft start cycle for the condition sensing apparatus and utilizing a Geiger tube wherein the inherent background count of the Geiger tube is utilized to initiate the safe start cycle, and thereby insure that all elements of the apparatus are in proper operating condition. Other modifications of the present invention will be apparent to those skilled in the art and it is intended that the scope of the present invention be limited solely by the scope of the appended claims.

I claim as my invention:

1. Condition sensing apparatus comprising; eigei tube condition sensing means, circuit means having an input and an output, means connecting said Geiger tube to said input, integrating means included within said circuit means and efiective to cause an output voltage to appear at said output only after said Geiger tube experiences a condition causing a sustained counting rate, first control means connected to said output to be energized by the voltage thereon, second control means adapted to be connected to a source of voltage to be energized thereby upon said first control means being energized, said second control means having means to maintain itself energized, and further means controlled by said second control means when de-energized connected to render said integrating means inoperative and thereby cause said first control means to be energized by the inherent background count of the Geiger tube to thereby initially energize said first control means and said second control means, whereupon said first control means is de-energized upon said integrata ing means being rendered operative due to energization of said second control means and can be re-energized only in the event of a sustained counting rate being experienced by said Geiger tube.

2. Condition sensing apparatus comprising; Geiger tube condition sensing means, integrating circuit means having an input and an output, means connecting said Geiger tube to the input of the said circuit means such that an output voltage appears at the output of said circuit means only upon said Geiger tube experiencing a sustained counting rate indicative of the presence or" a condition to be sensed, electrically energizable output means connected to the output of said circuit means to be energized by voltage thereon, condition simulating means connected to said circuit means to render said integratingcircuit means inoperative such that the random background count of said Geiger tube is effected to cause an output voltage to appear at the output of said circuit means to thereby energize said output means, further means controlled by said output means and effective when said output means is energized as a result of said background count to render said condition simulating means inoperative, further means controlled by said condition simulating means to maintain said condition simulating means inoperative once said condition simulating means is placed in an inoperative position to thereby render said integrating circuit means efiective such that an output voltage appears at the output of said integrating circuit means only upon said Geiger tube experiencing a sustained counting rate indicative of the condition to be sensed.

3. Condition sensing apparatus comprising; condition sensing means having a pair of electrodes disposed in an ionizable gaseous medium, voltage supply means, circuit means connecting said electrodes to said voltage supply means such that a pulse of current passes between said electrodes as an ionizing event passes through said gaseous medium, said condition sensing means experiencing a sustained counting rate of regular frequency upon being subjected to a condition to be detected and experiencing a random background counting rate caused by inherent ionizing events when not subjected to the condition; circuit means including electrical energy storage means, said circuit means having an input and an output, means connecting said condition sensing means to the input of said circuit means such that, so long as said electrical storage means is operative, a voltage appears at the output of said circuit means only upon said condition sensing means experiencing a sustained counting rate indicative of the presence of the condition to be detected; condition simulating means, means connecting said condition simulating means to said circuit means to render said electrical storage means inoperative when said simulating means is operative and thereby cause a voltage to appear at the output of said circuit means in accordance with the random background counting rate of said condition sensing means; output means, means connecting said output means to the output of said circuit means to be energized thereby, and further means controlled by said output means to render said condition simulating means inoperative upon said output means being initially energized and to maintain said condition simulating means inoperative thereafter, such that said output means is initially energized as a function of the random background count of said condition sensing means and is thereafter energized only as a function of the sustained counting rate indicative of the presence of the condition to be detected.

4. Condition'detecting apparatus comprising; a condition sensor having a pair of electrodes disposed in an ionizable gaseous medium, voltage supply means connected to said electrodes such that a pulse of electrical current passes through said ionizable gaseous medium upon an ionizing event causing ionization thereof, said ioniz'able gaseous medium being ionized at a random rate due to inherent background radiation and being ionized at a frequent rate due to the presence of a condition to be detected; circuit means having an input and an output and including an integrating network,'means connecting the input of said circuit means to said condition sensor such that a signal voltage is normally transmitted through said circuit means to the output thereof only upon said condition sensor experiencing a frequent rate of ionization, signal responsive means, means connecting said signal responsive means to the output of said circuit means, and means controlled by said signal responsive means and effective to render said integrating means initially inoperative such that said signal responsive means is initially energized in response to the background radiation whereupon said integrating means is subsequently rendered operative and said signal responsive means is thereafter energized only upon a frequent rate of ionization being experienced by said condition sensor.

5. Condition detecting apparatus having a Geiger tube condition sensor connected in circuit with integrating means and output means such that said output means is energized only upon said Geiger tube experiencing a sustained counting rate indicative of the condition to be detected but is not energized as a result of the inherent background count of the Geiger tube, the improvement comprising; means constructed and arranged to render said integrating means inoperative to thereby initially allow the output means to respond to the background count of said Geiger tube, and further means controlled by said output means upon said output means so responding to the background count of the Geiger tube to render said first named means inoperative and thereby render said integrating means operative and as a result thereof to de-energize said output means, whereupon said output means is thereafter energized only in response to a sustained counting by said Geiger tube indicative of the presence of the condition to be detected. 7

6. In a condition detecting apparatus of the type having an ionizable gaseous condition sensor connected in circuit with integrating means and output means, the improvement comprising; means controlled by said output means and effective upon initial energization of the con dition detecting apparatus to render said integrating means inoperative and thereby allow said output means to be energized in response to the inherent background count of said condition sensor, and further means controlled by said output means when energized arranged to render said integrating means operative whereupon said output means is de-energized and is re-energized only in response to a sustained counting by said condition sensor indicative of the presence of the condition to be detected.

7. Control apparatus comprising; a Geiger tube, voltage supply means connected to said Geiger tube to supply operating voltage thereto such that said Geiger tube produces a signal pulse upon an ionizing event passing through said Geiger tube, said Geiger tube having a random background counting rate producing random signal pulses in the absence of the condition to be detected and having a sustained counting rate producing sustained signal pulses. of frequent occurrence upon said Geiger tube being subjected to the condition to be detected, signal transmission means having integrating means and having an input connected to said Geiger tube, said signal transmission means being efiective to integrate the signal pulses derived from said Geiger tube and to produce an output voltage at the output thereofonly upon said Geiger tube being subjected to the condition to be detected, safe start means having an energized and a tie-energized condition, means controlled by said safe start means in the de-energized condition arranged to render said integrating means inoperative such that the background counting rate of said Geiger tube produces an output voltage at the output of said signal transmission means, output control means having an energized and a de-energized condition, means connecting said output control means to the output of said signal transmission means to be energized thereby, and means controlled by said output control means when in an energized condition to energize said safe start means such that said output control means is initially energized by the inherent background counting rate of said Geiger tube and said safe start means is then energized to render said integrating means operative and thereby cause said output control means to be de-energized and to be subsequently energized only upon said Geiger tube being subjected to the condition to be detected.

8. Condition detecting apparatus comprising; condition sensing means having a pair of electrodes disposed in an ionizable gaseous medium and constructed and arranged to cause a pulse of current to flow between said electrodes upon an ionizing event passing through said condition sensing means, said condition sensing means having an inherent background ionizing rate of random occurrence and having a sustained and regularly occurring ionizing rate upon said condition sensing means being subjected to a condition being detected, signal transmission means having signal storage means and having an input and an output, circuit means connecting the input of said signal transmission means to said condition sensing means such that an output signal appears at the output of said signal transmission means only upon said condition sensing means experiencing said sustained and regular ionizing rate, signal responsive control means connected to the output of said signal transmission means to be controlled thereby, safe start means connected to be controlled by said control means, said safe start means being connected to be energized upon said control means being initially energized and thereafter being maintained energized independent of said control means, and further means controlled by said safe start means when de-energized connected to said signal transmission means to render said signal storage means inoperative such that said control means is initially energized due to the inherent background ionization rate of said condition sensing means and is thereafter energized only upon said condition sensing means experiencing said sustained ionizing rate.

9. Condition detecting apparatus comprising; Geiger tube condition sensing means, signal transmission means having an input and an output and having means providing a signal storage function such that an output signal appears at the output thereof only upon a sustained input signal being applied to the input thereof, an output relay having a winding connected to the output of said signal transmission means and having a switch which is closed when said winding is energized, a safe start relay having a winding and a pair of switches, a first of which is closed when said safe start relay winding is de-energized and a second switch which is closed when said safe start relay winding is energized, circuit means controlled by said switch of said output relay connecting the winding of said safe start relay to a voltage source upon said output relay winding being energized, further circuit means controlled by the second switch of said safe start relay maintaining the winding of said safe relay connected to said source of voltage once said safe start relay winding is energized, and further circuit means controlled by the first switch of said safe start relay connected to said signal transmission means and arranged to render the signal storage function inoperative such that said output relay winding is initially energized due to the inherent background count of said Geiger tube but is then tie-energized and is subsequently reenergized only upon said Geiger tube experiencing a sustained counting rate indicative of the presence of the condition to be detected.

10. Condition detecting apparatus comprising; Geiger tube condition sensing means, said Geiger tube condition sensing means producing a random counting rate due to the inherent background count of the Geiger tube and producing a sustained counting rate when said Geiger tube is exposed to a condition to be detected, voltage transmission means including an integrating network, means connecting said Geiger tube to said voltage transmission means such that a voltage appears at the output thereof only upon said Geiger tube experiencing said sustained counting rate; an output relay having a winding connected to the output of said voltage transmission means and having a switch controlled thereby, said switch being closed when said winding is energized; a safe start relay having a Winding and a first and second switch controlled thereby, said first switch being closed when said winding is de-energized and said second switch being closed when said winding is energized; circuit means controlled by the first switch of said safe start relay connected to said voltage transmission means to render said integrating network inoperative such that the winding of said control relay is initially energized in accordance with the background counting rate of said Geiger tube; further circuit means controlled by the switch of said output .relay connecting the winding of said safe start relay to a source of voltage to thereby energize said safe start relay Winding upon said output relay winding being energized due to the inherent background counting rate of said Geiger tube, said last named circuit-means being effective to thereby render said integrating network operative and subsequently cause de-energization of the winding of said output relay, whereafter said output relay winding is re-energized only upon said Geiger tube experiencing said sustained counting rate; and further circuit means controlled by the second switch of said safe start relay maintaining the winding of said safe start relay connected to said source of voltage once the winding thereof is initially energized.

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